Data migration may be necessary when a medium or a media drive device reaches the end of its life. Alternatively, data migration may be performed for operational reasons, such as for switching to a more cost-effective medium.
During data migration, it is desirable that frequently accessed data be recorded at a position advantageous in terms of performance. For example, when a plurality of pieces of data are accessed as a single data group, a piece of data recorded at a disadvantageous position in terms of performance may affect overall performance of access to the data group.
For migration of data recorded in one medium to itself (i.e., another region in the same medium) or to another medium, there are known techniques that improve performance by considering frequency of data access.
Japanese Unexamined Patent Application Publication No. 2007-164650 discloses a technique in which a rearrangement plan is created such that a frequently accessed logical volume is arranged in a RAID group in a long-time energization mode, a less frequently accessed logical volume is arranged in a RAID group in a first short-time energization mode, and a least frequently accessed logical volume is arranged in a RAID group in a second short-time energization mode. Then, execution of the rearrangement plan is determined based on access frequency to each RAID group.
Moreover, Japanese Unexamined Patent Application Publication No. 8-263335 discloses a technique in which access frequency to each file including a plurality of blocks is stored, a plurality of files related to each other and accessed with frequency not less than a predetermined value are extracted as a single group, data of each block in each of the extracted files is read from an optical disk, and the read pieces of data are sequentially written to consecutive or neighboring blocks on the optical disk.
Furthermore, Japanese Unexamined Patent Application Publication No. 11-31376 discloses a technique in which memory is provided in a cartridge in a tape cassette, and information about access frequency to each partition is recorded in the memory. To duplicate the tape, information about access frequency is read, and pieces of data are recorded in descending order of access frequency, starting at the beginning of the tape.
According to the aforementioned publications, the access frequency is recorded for a data group (e.g., logical volume, file, or partition) that includes a plurality of pieces of data. The data group may include data that has been present for a long time as well as data that has been created recently. For pieces of data that have been created at different times, real access frequency cannot be obtained when access frequency is determined with reference to the time axis. Therefore, the known techniques disclosed in the aforementioned publications are disadvantageous in that it is not possible to improve performance by considering real access frequency in data migration.